There are a number of industries that generate large quantities of biomass. Two examples include the forest products and agricultural industries. For example, in the forest product industries, large quantities of biomass are generated, including sawdust, bark, twigs, branches, and other waste wood. Likewise, in the agricultural industries, each crop cycle results in large quantities of biomass, including bagasse, corn cobs, rice hulls, and orchard and vine trimmings. Additional biomass waste that is generated includes sludge and manure. Despite the large quantities that are produced, this waste biomass cannot be easily utilized.
To alleviate problems associated with the disposal of biomass, biomass has been used heretofore for power generation. Additionally, because biomass is a renewable resource and because biomass releases the same amount of carbon to the atmosphere as it does when it decomposes naturally, the use of biomass for power generation may address several problems with conventionally fossil fuels.
One technique that has been developed for using biomass for power generation is gasification. In gasification, biomass is converted to a combustible gas, which may then be used to generate electricity, for example in a gas turbine. However, when employed in small-scale power system (e.g., less than about 10 megawatts), these gasification techniques typically have lower thermal efficiencies and higher capital and operating costs than the direct-fired gas turbine power systems discussed below. Likewise, techniques using solid fuels such as biomass for steam generation also typically have lower thermal efficiencies and higher capital and operating costs than the direct-fired gas turbine power systems discussed below.
As an alternative to gasification and steam generation techniques, power systems that generate electricity by driving gas turbines, using solid fuels such as biomass, have also been used. Gas turbine power systems that operate on solid fuel may be designed as either indirect-fired or direct-fired systems. These systems typically have several primary components, including an air compressor, a furnace or combustor, a turbine, and an electric generator. The electric generator and air compressor are driven by energy created by expansion of hot compressed air through the turbine. This hot compressed air for expansion across the turbine is generated by compressing air in the compressor and heating the resultant compressed air with thermal energy generated by the furnace or combustor.
In indirect-fired systems, the furnace or combustor typically operates as a separate functional unit apart from a functional unit containing the air compressor and the turbine. This design for indirect-firing protects the gas turbine from corrosive effluents and particulate matter typically present in the hot exhaust gases from a furnace or combustor operating on biomass by use of a high temperature heat exchanger. In the high temperature heat exchanger, ducts containing the compressed air from the compressor may be placed in close proximity to ducts bearing highly heated exhaust gases from the furnace or combustor, resulting in exchange of heat from the hot exhaust gases to the compressed air. This heated and compressed air then drives the turbine which in turn drives the air compressor and electric generator. In addition to higher capital costs and operating costs, these indirect-fired systems have lower thermal efficiencies than direct-fired system.
In direct-fired systems, the solid fuel is burned in a pressurized combustor and the heated effluent gases from the combustor are vented directly into the turbine. The combustor is part of an integrated, pressurized unit that includes both the compressor and the turbine. In many instances, gas cleaning equipment may be employed between the combustor and turbine to reduce the entry of corrosive effluents and particulate matter into the turbine.